Samer Kawak Sandbox 3

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a very important enzyme in the production of energy. This enzyme catalyzes the sixth step in the process of breaking down glucose into energy, also known as glycolysis. Though this is its main function, GAPDH has been shown to perform other functions including transcription activation, initiation of apoptosis, and ER to Golgi apparatus vesicle transportation. However, this page will focus on GAPDH’s role in glycolysis. GAPDH most commonly exists as what looks to be a dimer. Interesting though, the two monomers of the enzyme are not exactly the same. While one side consists only of parallel and antiparallel beta-sheets, the other monomer is made up of both beta-sheets and alpha helixes. Though each monomer does not have to exact same sequence, each does contain replicate active sites and function. This is consistent with the following SCOP information:

Class: Alpha and beta proteins (a/b) Fold: NAD(P)-binding Rossmann-fold domains Superfamily: NAD(P)-binding Rossmann-fold domains Family: Glyceraldehyde-3-phosphate dehydrogenase-like, N-terminus domain Protein: Glyceraldehyde-3-phosphate dehydrogenase Species: Human

The specific reaction that GAPDH catalyzes is shown below:

GAP + NAD+ + Pi +GAPDH <==> 1,3-bisphosphoglycerate + NADH

The mechanism of the glycolysis reaction is fairly straight forward. After the aldehyde enters the active site (highlighted in green), the sulfhydryl group from Cystine 151 attacks the nucleophilic carbon to form a thiohemiacetal. This intermediate undergoes oxidation due to a hydride transfer to a nearby NAD+ forming a thioester. From here, a phosphate group enters and attacks the same carbonyl while at the same time it is separated from the cystine by the protonated Histidine 178 group. This produces the desired 1,3-bisphosphoglycerate. Though cystine-151 and histidine-178 are direct contributers to the catalytic process, other residues also influence the activity of this enzyme indirectly. Thr-210 and Arg-233 are two such residues that contribute to the binding of the reactants rather than the catalytic mechanism. Regulation of GAPDH occurs through its coupling with the PGK reaction. This coupling is needed due to the slightly positive delta G of the glycolysis. The larger negative delta G of the PGK reaction results in the following overall net reaction with a delta G of -12.1 kJ/mol:

GAP + Pi + NAD+ + ADP ==> 3PG + NADH + ATP

Additional Resources
For additional information, see: Carbohydrate Metabolism